How healthy are urban horticultural crops? A study of the accumulation of heavy metals in vegetables grown in Tehran

Urbanization and population growth and their rapid development across the world have led to a major demand for the food security and self-sufficiency in terms of food production in many cities. Accordingly, activities based on urban agriculture using different traditional methods (e.g. planting on the ground surface with a soil bedding) or modern ones (e.g. rooftop gardening with hydroponic cultivation methods) have emerged and numerous studies regarding these methods have been accomplished. Likewise, a large number of farms in urban areas have come into existence worldwide. Nonetheless, there are yet various challenges in developing this type of agriculture. Moreover, only a few studies have been carried out to monitor the health and hygiene condition of the food produced in such places, especially in terms of air pollutant accumulation. The objective of this research was to study the hygiene of the plants cultivated in an urban environment (rooftops and courtyards), and to determine the accumulation rates of pollutants in the eggplant fruit (Solanum Melongena var. depressum (L.)) and bell peppers (Capsicum fruitescens var. grossum (Mill.)). Two types of vegetables; bell pepper and eggplant were cultivated in the rooftop and courtyard of a five-storey building in the 7th district of Tehran. The seeds were planted on the basis of a completely randomized design system using six replications. In order to compare the health of these fruits with those of the fruits cultivated in the countryside, similar samples of the same products were grown on farms located in the environs of Varamin and Karaj cities. Densities of heavy metals such as molybdenum, chrome, copper and manganese (Mn, Mo, Cr and Cu) found in bell pepper and eggplant fruit cultivated in the urban areas of Tehran were compared with amounts found in fruits cultivated on the farms located outside towns. Additionally, the amounts of the heavy metals lead and cadmium (Cd, Pb) in the samples related to the city of Tehran were compared to European Union Standards and the Iranian National Standard. The sample digestion was carried out using microwave and, in order to analyse and measure the results, the ICP-MS device was used. Data was analysed using SAS and Microsoft Excel software and the comparison of means was done through standard error. The resultant data demonstrate that the densities of heavy metals in the most of the fruits grown in city showed were significantly higher (in the range of 5%) than those of plants grown in countryside, except for the content of chrome in eggplant which is higher in the samples from farms located in the countryside than those of both urban areas. Moreover, the heavy metal pollution of copper in both eggplant and bell pepper from the farms located in countryside were higher than the samples of the rooftop. The chrome density in the eggplants cultivated in farmland was 1.56 times more than that of the courtyard plants, and 1.58 times more than those grown on the rooftop. Densities of copper in the eggplant and bell pepper of countryside farm are, respectively, 1.22 and 1.24 times more than the rooftop samples. Furthermore, except for lead content (Pb) being observed at the three different levels of 0.18, 0.63 and 0.14 milligrams per kilogram of dry weight (mg/kgDW) in some samples, respectively, for rooftop eggplants, rooftop bell peppers and courtyard bell peppers which showed significant variances to the permissible limits (in the range of 5%), other samples indicated acceptable amounts of lead and cadmium with respect to the European Union Standard and Iranian National Standard (under the limits of 5%). In general, it can be concluded that all the samples cultivated in the urban areas of 7th district of the city of Tehran were healthy in terms of allowable amounts of the heavy metal cadmium. These density ratios fully complied with international standards, and were similar to those of the samples cultivated in the countryside. In contrast, most of the aforementioned samples contained impermissible amounts of the heavy metal lead. Likewise, the densities of other heavy metals included in the products of urban farms were much higher than those grown on the city exteriors. This difference may well be the impact of the effects of heavy traffic flow, geographic position and the unsuitable climatic conditions on the air quality of urbanized areas.